HONG KONG -- A new law in Hong Kong that requires drivers to turn their parked cars’ engines off comes into effect today. But it’s expected to be an environmental letdown.

The idling ban is 10 years in the making from when it was first proposed. The city is notorious for its smog problem, and heavy road traffic is considered a contributing factor.

The government has been using public-service announcements for years to ask drivers to switch off their idling engines, but real legislation has finally come into effect.

For every 60 minutes that a vehicle is stationary, the driver is only allowed to have the engine on for three minutes.

The ones who will be most affected by the ordinance are personal drivers who frequently wait in a parked car while running the engine to keep the air-conditioner going. Leaving their engines on for more than three minutes can now result in a $40 ticket.

But the rule does not apply to all motorists—far from it. The long list of exceptions includes taxis waiting at taxi stands and buses with passengers on board.

It also exempts all drivers when the weather is very hot or very rainy; drivers’ main complaint about idling legislation is that Hong Kong’s extreme summer heat endangers their health—and their lives—if they have to go without air-conditioning.

Edwin Lau, director of general affairs at Friends of the Earth in Hong Kong, called it “a watered-down legislation.” He said it will function more as a reminder for drivers who might shift their attitudes about idling engines, rather than a measure to improve air quality.

“This legislation is always good to have, even if the enforcement will be a bit difficult,” he said. For the legislation to have a great environmental impact, Lau said, some of the exemptions would need to be removed.